SAFETY AND IMMUNOGENICITY OF A COLD-ADAPTED INFLUENZA-A (H1N1) REASSORTANT VIRUS-VACCINE ADMINISTERED TO INFANTS LESS-THAN-6 MONTHS OF AGE

A safe and effective influenza vaccine is needed to prevent serious in fluenza illness in infants younger than 6 months of age. The purpose o f this study was to determine whether two doses of the cold-adapted (c a) influenza A reassortant vaccine would be safe and immunogenic in th is age group. In the first part of this study, infants received two do ses of 10(5) or 10(6) 50% tissue culture-infectious dose (TCID50) of t he ca influenza vaccine separately from routine immunizations. In the second part of this study two 10(6) TCID50 doses of the ca influenza v accine were given with routine immunizations at 2 and 4 or 2 and 6 mon ths of age. The ca influenza vaccine was well-tolerated by participant s in both parts of this study. Two doses of the ca influenza vaccine w ere immunogenic in infants who received them separately from routine i mmunizations; 83% of vaccinees developed protective titers of serum he magglutination-inhibition (HAI) antibody. In contrast, when the ca vac cine was administered with routine immunizations, protective HAI antib ody titers were induced in only 20% of those immunized at 2 and 4 mont hs of age and 50% of those immunized at 2 and 6 months of age. There w ere no statistically significant associations between HAI antibody res ponse to ca influenza vaccination and dose schedule, presence of passi vely acquired maternal HAI antibody, ethnic group or breast-feeding st atus, Young age at the time of first immunization, however, appeared t o correlate with decreased response to the hemagglutinin antigen of th e influenza A virus. Further studies are needed to determine whether a larger dose of vaccine (10(7) TCID50) could be used safely to improve the serum antibody response to influenza in very young infants.